The comparative structures of the sympathetic and parasympathetic divisions are listed in tables 9.2 and 9.3. It should be noted that most parasympathetic fibers do not travel within spinal nerves, as do sympathetic fibers. As a result, cutaneous effectors (blood vessels, sweat glands, and arrector pili muscles) and blood vessels in skeletal muscles receive sympathetic but not parasympathetic innervation.

Four of the twelve pairs of cranial nerves (described in chapter 8) contain preganglionic parasympathetic fibers. These are the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. Parasympathetic fibers within the first three of these cranial nerves synapse in ganglia located in the head; fibers in the vagus nerve synapse in terminal ganglia located in widespread regions of the body.

■ Figure 9.3 The pathway of sympathetic neurons. The preganglionic neurons enter the sympathetic chain of ganglia on the white ramus (one of the two rami communicantes). Some synapse there, and the postganglionic axon leaves on the grey ramus to rejoin a spinal nerve. Others pass through the ganglia without synapsing. These ultimately synapse in a collateral ganglion, such as the celiac ganglion.

■ Figure 9.3 The pathway of sympathetic neurons. The preganglionic neurons enter the sympathetic chain of ganglia on the white ramus (one of the two rami communicantes). Some synapse there, and the postganglionic axon leaves on the grey ramus to rejoin a spinal nerve. Others pass through the ganglia without synapsing. These ultimately synapse in a collateral ganglion, such as the celiac ganglion.

224 Chapter Nine

Superior mesenteric ganglion

First lumbar sympathetic ganglion

Pelvic sympathetic chain

Superior mesenteric ganglion

First lumbar sympathetic ganglion

Pelvic sympathetic chain

Diaphragm

Celiac ganglion

Renal plexus

Aortic plexus

Inferior mesenteric ganglion

Diaphragm

Celiac ganglion

Renal plexus

Aortic plexus

Inferior mesenteric ganglion

■ Figure 9.4 The collateral sympathetic ganglia. These include the celiac ganglion and the superior and inferior mesenteric ganglia.

Table 9.2 The Sympathetic Division

Parts of Body Innervated

Spinal Origin of Preganglionic Fibers

Origin of Postganglionic Fibers

Eye

C8 and Tl

Cervical ganglia

Head and neck

Tl to T4

Cervical ganglia

Heart and lungs

Tl to T5

Upper thoracic (paravertebral) ganglia

Upper extremities

T2 to T9

Lower cervical and upper thoracic (paravertebral) ganglia

Upper abdominal viscera

T4 to T9

Celiac and superior mesenteric (collateral) ganglia

Adrenal

Tl0 and Tll

Not applicable

Urinary and reproductive systems

Tl2to L2

Celiac and interior mesenteric (collateral) ganglia

Lower extremities

T9 to L2

Lumbar and upper sacral (paravertebral) ganglia

The oculomotor nerve contains somatic motor and parasympathetic fibers that originate in the oculomotor nuclei of the midbrain. These parasympathetic fibers synapse in the ciliary ganglion, whose postganglionic fibers innervate the ciliary muscle and constrictor fibers in the iris of the eye. Preganglionic fibers that originate in the pons travel in the facial nerve to the pterygopalatine ganglion, which sends postganglionic fibers to the nasal mucosa, pharynx, palate, and lacrimal glands. Another group of fibers in the facial nerve terminates in the submandibular ganglion, which sends postganglionic fibers to the sub-mandibular and sublingual salivary glands. Preganglionic fibers of the glossopharyngeal nerve synapse in the otic ganglion,

■ Figure 9.5 The path of the vagus nerves. The vagus nerves and their branches provide parasympathetic innervation to most organs within the thoracic and abdominal cavities.

which sends postganglionic fibers to innervate the parotid salivary gland.

Nuclei in the medulla oblongata contribute preganglionic fibers to the very long tenth cranial, or vagus nerves (the "vagrant" or "wandering" nerves), which provide the major parasympathetic innervation in the body. These preganglionic fibers travel through the neck to the thoracic cavity and through the esophageal opening in the diaphragm to the abdominal cavity (fig. 9.5). In each region, some of these preganglionic fibers branch from the main trunks of the vagus nerves and synapse with postganglionic neurons located within the innervated organs. The preganglionic vagus fibers are thus quite long; they provide parasympathetic innervation to the heart, lungs, esophagus, stomach, pancreas, liver, small intestine, and the upper half of the large intestine. Postganglionic parasympathetic fibers arise from terminal ganglia within these organs and synapse with effector cells (smooth muscles and glands).

Preganglionic fibers from the sacral levels of the spinal cord provide parasympathetic innervation to the lower half of the large intestine, the rectum, and to the urinary and reproductive systems. These fibers, like those of the vagus, synapse with terminal ganglia located within the effector organs.

Parasympathetic nerves to the visceral organs thus consist of preganglionic fibers, whereas sympathetic nerves to these organs contain postganglionic fibers. A composite view of the sympathetic and parasympathetic systems is provided in figure 9.6.

Test Yourself Before You Continue

1. Using a simple line diagram, illustrate the sympathetic pathway (a) from the spinal cord to the heart and (b) from the spinal cord to the adrenal gland. Label the preganglionic and postganglionic fibers and the ganglion.

2. Explain what is meant by the mass activation of the sympathetic system and discuss the significance of the term sympathoadrenal system.

3. Using a simple line diagram, illustrate the parasympathetic pathway from the brain to the heart. Compare the parasympathetic and sympathetic divisions in terms of the locations of the pre- and postganglionic fibers and their ganglia.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.